(1) Field of the Invention
The present invention generally relates to a towed acoustic sensor array. More particularly, this invention relates to a method for operating a towed acoustic sensor array.
(2) Description of the Prior Art
Naval vessels and geophysical exploration vessels routinely deploy towed acoustic arrays for analyzing undersea structures and objects. Improving signal to noise ratio is critical for these array. This can be increased by decreasing noise sources in the towed acoustic array. A standard towed array deployment is shown in FIG. 1. A tow vessel 10 is provided having a tow cable 12 joined to a towed array 14. Towed array 14 is below a water surface 16. As tow vessel 10 moves in the direction indicated by arrow 18 with velocity V, the towed array 14 is subjected to surface friction along length L1 creating a force F in the direction shown by arrow 20. This results in a tension in towed array 14. It is understood that tow vessel 10 can be a surface or subsurface vessel.
Flow noise is an important component of towed array noise. Flow noise in towed arrays can result from bulge waves and extensional waves propagating in the hose wall, and also from convective energy due to the turbulent boundary layer. These all have a well defined distribution of energy across stream-wise wavenumber at a given frequency, and also a well defined propagation speed.
The turbulent boundary layer developed over the surface of a towed array and vortex shedding induced during turns, are primary sources of flow noise. Standard towed array design is aimed at filtering energy from the turbulent boundary layer using a combination of hydrophone size, hydrophone grouping, and the stand-off distance between the hydrophone and the hose wall of the array. These methods are effective for filtering energy which exists at higher wavenumbers than acoustic energy at low wavenumbers.
FIG. 2 is a diagram of a prior art towed array section 22. Towed array section 22 has a hose wall 24. The hollow interior 26 of hose wall 24 is typically filled with non-conducting oil. Hose wall 24 has an external diameter D as indicated in the figure. A plurality of strength members 28 extend through the hollow interior 26. (For clarity, hidden views of strength members 28 are omitted.) A communications line 30 also extends through the interior 26. Communications line 30 can be either an electrical line or a fiber optic line. Hydrophones 32 are joined to communication line 30 and positioned at intervals within hollow 26. As towed array section 22 is towed through the water a turbulent boundary layer 34 of thickness δ forms around the exterior of hose wall 24.
Therefore, there is a need for reducing flow noise in towed acoustic array.